In combined cycle power generation, first of all, a gas turbine is driven using natural gas or the like as fuel, thereby carrying out the first power generation. Next, a heat recovery steam generator recovers exhaust gas of the gas turbine and generates steam. Then, a steam turbine is driven by means of the steam, thereby carrying out the second power generation. Combined cycle plants are power generation plants for executing the combined cycle power generation.
When a combined cycle plant is started up, a standby load for the gas turbine is set in accordance with a metal temperature of the steam turbine. For example, when the metal temperature of the steam turbine is equal to or lower than 200° C., a cold start-up is conducted. The standby load for the gas turbine is set to 10% and the gas turbine is started up. Meanwhile, when the metal temperature of the steam turbine is equal to or higher than 400° C., a hot start-up is conducted. The standby load for the gas turbine is set to 30% and the gas turbine is started up. In addition, when the metal temperature of the steam turbine is within a range from 200° C. to 400° C., the standby load for the gas turbine is set to 20% and the gas turbine is started up. Then, after the gas turbine is started up and a set standby load is retained thereon, when steam generated by means of exhaust gas reaches a predetermined temperature and predetermined pressure, the steam is supplied to the steam turbine, and the load on the gas turbine is increased. More specifically, when a mismatch between the steam temperature in an inlet of the steam turbine derived out from the temperature and the pressure of steam on an outlet side of the heat recovery steam generator, and the metal temperature of the steam turbine is reduced, and when a condition that the degree of superheat is sufficiently ensured is satisfied, steam starts to be supplied to the steam turbine.
Incidentally, there are demands for starting up a combined cycle plant at a standstill in an early stage and supplying electric power. Here, a gas turbine alone is capable of increasing the load at a relatively high load increasing rate. However, in a steam turbine, due to the restriction of thermal stress, the load is required to be increased at a load increasing rate lower than that in the gas turbine. That is, in the combined cycle plant, even though the load can be increased to a standby load for the gas turbine relatively quickly, the loads on both the gas turbine and the steam turbine are required to be increased at a low speed after steam starts to be supplied to the steam turbine. Accordingly, compared to a case of the gas turbine alone, it takes time to increase the load. Thus, the smaller the standby load for the gas turbine, the larger a load zone through which the loads on the gas turbine and the steam turbine are simultaneously increased. Therefore, the time for increasing the load on the entire combined plant is further lengthened.
Meanwhile, when the above-described combined cycle plants in the related art are started up, the standby load for the gas turbine with respect to a metal temperature range of the steam turbine is fixed for each start-up mode. Therefore, the standby load for the gas turbine changes at the border between the metal temperature ranges of the steam turbine. For example, when the metal temperature of the steam turbine is 195° C., a cold start-up is conducted, and the standby load for the gas turbine is set to 10%. When the metal temperature of the steam turbine is 205° C., a warm start-up is conducted, and the standby load for the gas turbine is set to 20%. In this case, although the difference between the metal temperatures of the steam turbine is 10° C., which is insignificant, when the metal temperature of the steam turbine is 195° C., the standby load for the gas turbine is set to 10%. Thus, a slight difference between the metal temperatures of the steam turbine makes the standby load for the gas turbine change significantly, thereby leading to a problem in that the time for starting up the combined cycle plant is lengthened.
In addition, when the above-described combined cycle plants in the related art are started up, an increasing load rate of the steam turbine during a start-up is fixed for each start-up mode. Therefore, the increasing load rate of the steam turbine changes at the border between the metal temperature ranges of the steam turbine. For example, when the metal temperature of the steam turbine is 195° C., a cold start-up is conducted, and the increasing load rate is set to be relatively low. When the metal temperature of the steam turbine is 205° C., a warm start-up is conducted, and the increasing load rate is set to be relatively high. In this case, although the difference between the metal temperatures of the steam turbine is 10° C., which is insignificant, when the metal temperature of the steam turbine is 195° C., the increasing load rate of the steam turbine is set to be on the low side. Thus, a slight difference between the metal temperatures of the steam turbine makes the increasing load rate of the steam turbine change significantly, thereby leading to a problem in that the time for starting up the combined cycle plant is lengthened and it requires time.
The present invention has been made in order to solve the above-described problems, and an object thereof is to provide a combined cycle plant, a device for controlling a combined cycle plant, and a method for starting up a combined cycle plant that can shorten a time for starting up the combined cycle plant.